How do I validate purchase in whmcs integration

Hi guys, I hope you are doing great!

I have WHMCS integration in my website and there's a problem... When someone makes a payment using your plugin, it always returns to the invoice with a message Not Paid.

I don't know if this is due to the fact that the return URL doesn't match but if I make the payment through WHMCS directly there's no problem at all.

Can you please help me?

Thanks